A variety of surface treatments are applied to metal materials in order to improve various characteristics such as abrasion resistance, corrosion resistance, and strength. Surface treatments include carburization, sulfurizing, nitriding, and carbonitriding. In other cases, a coating may be provided by means of, for example, the physical vapor deposition (PVD) method, the chemical vapor deposition (CVD) method, the plating, and the anodic oxidation.
For example, the means for hardening the surface of a Zn alloy such as Zn—Al—Sn alloys is exemplified by a direct electroless nickel plating method disclosed in Japanese Patent No. 2832224. In this method, a die composed of the Zn alloy is immersed in an electroless nickel plating solution containing an organic acid nickel salt or the like to form a nickel coating on the surface of the die.
According to Japanese Patent No. 2832224, the Zn alloy coated with nickel coating as described above is satisfactory in abrasion resistance and corrosion resistance.
However, in any one of the methods as described above, the improvement in various characteristics is limited to the surface of the metal material. For example, in the case of the nitriding and the carburization, the element is diffused only by several tens μm, or about 200 μm at the maximum from the surface of the metal material. It is difficult to improve various characteristics in other regions disposed more internally than the foregoing.
This inconvenience similarly occurs in the coating formation as represented by the invention disclosed in Japanese Patent No. 2832224 described above. Further, in this case, the interface exists between the coating and the metal material. Therefore, when the coefficient of thermal expansion of the coating is extremely different from that of the metal material, the film may be exfoliated by repeating the heating and the cooling.
Further, in certain metal materials such as Zn alloys, Al alloys, and Ti alloys which have an oxide film formed quickly on the surface, the method of forming the coating may be limited to the plating, the anodic oxidation, or the like. By such methods, the thickness of the coating is small. Therefore, various characteristics cannot be sufficiently improved.